Leishmaniasis caused by Leishmania parasites represents an important public health problem that currently affects more than 350 million people around the world, Whereas a key role of Type II interferon (IFN?) in resistance to Leishmania is well established, according to the current paradigm, Type I interferons (such as IFNa species) are not considered of major importance. This line of thinking stems from the data that IFNa receptor subunit 1 (IFNAR1)-null mice are not overly sensitive to Leishmania infection compared to wild type animals. However, an alternative possibility that Leishmania may simply inactivate IFNAR1 has not been explored. Here we propose a paradigm-shifting hypothesis that Leishmania promastigotes are capable of down regulating IFNAR1 levels via expression of parasite casein kinase 1 (L-CK1) that leads to increased Ser535 phosphorylation of human IFNAR1 (Ser526 in mice), which triggers the recruitment of [unreadable]-Trcp E3 ubiquitin ligase followed by IFNAR1 ubiquitination and degradation. This L-CK1-mediated down-regulation of IFNAR1 is important for impairing IFNa signaling (including IFNa-induced expression of inducible nitric oxide synthase, iNOS) to decrease the resistance against Leishmania infection. This hypothesis is based on our preliminary data indicating that: (i) down regulation of IFNAR1 that occurs via its Ser535/526 phosphorylation- dependent ubiquitination and degradation limits the extent of cellular responses to IFNa, (ii) expression of L- CK1 leads to increased Ser535/526 phosphorylation and degradation of IFNAR1, (iii) shed L. major promastigote kinase (previously identified as L-CK1) is capable of phosphorylating IFNAR1, (iv) L. major infection down regulates IFNAR1 levels in human dendritic cells; and (v) either L. major infection or expression of L-CK1 inhibits the extent of cellular responses to IFNa. To test our hypothesis we propose: (1) To determine the effect of L. major infection and expression/activity of L-CK1 on the down regulation of IFNAR1. We will investigate whether Ser535/526 phosphorylation of IFNAR1, recruitment of [unreadable]-Trcp E3 ubiquitin ligase, IFNAR1 ubiquitination and degradation are increased in response to expression of L-CK1 or L. major infection (under the conditions that permit or disallow L-CK1 activity and IFNAR1 phosphorylation) using transgenic L. major L-CK1 haploinsufficient and mutant knock-in strains as well as host cells expressing a stabilized IFNAR1 mutant; (2) To determine the role of L-CK1-induced IFNAR1 degradation in modifying the cellular responses to IFNa and in resistance to L. major infection. We will determine whether L. major infection or L-CK1 expression affects the magnitude and duration of IFNa signaling and expression of IFN-stimulated genes (ISG) that play an important role in anti-Leishmania defense (e.g., iNOS). We will further examine the role of IFNAR1 phosphorylation in these responses via generating the knock-in mice that harbor an IFNAR1 point mutant (S526A) that should render IFNAR1 resistant to L-CK1-mediated phosphorylation and will examine the sensitivity of these mice to L. major infection. Single celled parasites of the genus Leishmania cause a dangerous disease (leishmaniasis) that currently affects more than 350 million people around the world, including the US troops in the Middle East. Treatment of leishmaniasis is impeded by a limited arsenal of drugs, their high toxicity, and emerging resistance of parasites to these agents. Hence, it is imperative to identify novel treatment targets, in particular, those that affect the innate resistance of cells to Leishmania promastigotes. Our studies (aimed at identification and characterization of Leishmania casein kinase 1 as a major regulator of cellular responses to such innate resistance regulators as Type I interferons) should gain the insight on the mechanisms by which Leishmania may evade the defensive mechanisms of mammalian cells and, potentially, identify the novel targets for the therapy against leishmaniasis. [unreadable] [unreadable] [unreadable]